Whether you are interested in water cooling your PC, riveting together your DIY nuclear sub, or just want something different to chat about in the bar, you ought to know about Galvanic Corrosion. You may not think that you do, but you do.

People kept mentioning it to me, and I hate it when other people seem to know more than I do - so I've been doing some reading up. Here, for you delight and entertainment is what I've found out:

(Please bear in mind that this isnâ€™t supposed to be a detailed technical discussion that would hold up at a marine engineerâ€™s conference â€“ itâ€™s just meant to hold up at LAN parties and dinner parties. If you notice any glaring mistakes, then please have a quiet word with me at [email protected], or on the private message system here)

What is Galvanic Corrosion, and why is it a bad thing?

It isnâ€™t necessarily a bad thing. The first ever batteries worked on the principal of Galvanic Corrosion: If you put two dissimilar metals (like steel and aluminium) in an electrolyte (a liquid that conducts electricity, like water), a volt meter will show you that a current flows between them.

The first ever batteries were made by sandwiching layers of different metals with salt water soaked stuff â€“ the so called â€œWet Cellâ€

_________________Charlie

Last edited by charliek on Tue Feb 19, 2008 10:00 pm, edited 3 times in total.

Any significant corrections/additions should be edited into Charliek's first post, so that people will get the best information at the very top of the thread.

I'll add a minor amendment:

Most of what WC'ers online describe as, and show pictures claiming to be galvanic corrosion, isn't. It's just plain oxidation. If your copper just turns green...its not galvanic corrosion, its just plain old corrosion.

I'm generally greatful when someone goes to similar effort for me and I was going to do the research anyway, so...

Ed - I don't know the product you mention, but if it's electrically non-conductive, then yes it'll prevent galvanic corrosion, as for regular corrosion (rust, copper verdigris, etc...) its lack of conductivity won't make any difference to that. Its other chemical properties might, in either direction. Which ties in with:

Rusty - exactly: Galvanic corrosion effect either speeds up, or slows down, existing corrosion effects. However the good news is that the 'sacrificial anode' method of galvanically protecting your other metals will effectively slow down corrosion on all of them, except for the sacrifical anode (assuming you chose one that's more anodic than the rest - Zinc for us mortals).

Next time I bump into a chemist or a physicist at a party (it happens you know), I'll ask him or her to look this up and down and comment. (unless, of course, she's a 'her' and there are other things that I decide to discuss with her)

Difficlt to do HS... The anode and cathode are connected by the conductive coolant... The coolant is even arranged in a loop so you have a potentially complete circuit!

Overall a good article, I would add however that anti-corrosives, coatings, etc. in part because of the wearing out issue, won't completely stop GC, but merely slow it down. Now if one slows it down enough that the hardware lasts longer than the effective service life of the PC, it could be argued that this is "good enough", but I tend not to buy that stance since 1. I usually keep my PC's far longer than most folks, and 2. WC hardware tends to be more 'reusable' than other PC components so you might well find yourself wanting to use WC hardware long after the PC it was originally installed on has been retired.

I didn't notice any mention of eliminating the electrical connection between the anode and cathode. This should cause a substantial reduction in galvanic action.

I kind of touched on this in my reply to Ed. Yes, if you eliminate the conductive path between the metals concerned, you will eliminate the effects of galvanic corrosion. However...

a) Given that we're discussing, in most cases, two lumps of metal connected by a path of liquid (see also 'connections' below), it is likely to be prohibitively difficult or expensive to make it totally non-conductive - also, even if you make your liquid non-conductive you are not necessarily making it non-corrosive or non-oxidising, which brings me to...

b) You may not want to eliminate the effects of galvanic corrosion. You won't necessarily eliminate the effects other corrosion (see above), and you can use the galvanic corrosion effect to your advantage by setting up a 'sacrifical anode' (a lump of disposable metal that is more anodic than the ones you want to preserve), that will corrode faser than - and thereby protect - your important lumps of metal.

Connections

HammerSandwich wrote:

Can the electrolyte serve as the return also? Or does GC require a separate connection? Charliek's references indicate the second

That's a good question, and I'm not 100% sure of the whole answer (bear in mind that I'm no expert on any of this - I've just been reading)

What I am sure of is the fact that you don't need a 'loop of water' to create a 'send and return' circuit like you'd expect in a traditional electrical circuit.

If you get a bucket of tap water, and lob in a lump of zinc and a lump of silver, there'll be an electrical current between them and the process of galvanic corrosion will corrode the zinc faster than it would have normally corroded, and the silver slower than normal.

The process involves the exchange of metallic ions in the water (the electrolyte), there are positive 'cations' and negative 'anions' that are attracted to the anode and the cathode, respectively, through the electrolyte. This is in danger of getting over-technical, and it has been a long time since my 'O'Level chemistry so, suffice it to say that the current flows in both directions through the water.

Galvanic corrosion, remember, is in addition to all the normal reactions one would expect. A lump of iron in water will rust, because water is an oxydising agent - it forms iron oxide, rust, on the iron, corroding it. Chuck a lump of magnesium in, and there develops a galvanic relationship between the two - the zinc is more anodic, and so will corrode faster than it would have on its own, the iron is more cathodic, so its rusting will slow down. Substitute silver for the zinc, and the opposite will occur - the iron is now the more anodic, and will rust like anything while the silver will remain smug and shiny for longer than it would have on its own. These processes all use the same water - it is at the same time an oxydising agent, and a conducter (an electrolyte).

I'm guessing that you found the reference to the sacrificial anode, in which it was attached to the metal that it was there to preserve by a wire, as well as by the water. I am unclear as to why this should be necessary, there is already a connection because they are in the same electrolyte after all. My guess is that the wire is less resistive than the water - i.e. it makes the path between the two metals the 'path of least resistance' - and thereby guarantees that the anode-cathode relationship between them is as efficient as possible.

To put it another way, because the zinc is connected to the copper by a wire, the copper will concentrate all of its cathodic energy on eroding the easily-available sacrifical zinc, and wont touch the aluminium at all.

So it would be beneficial to, say, stick in a piece of zinc to replace a piece of tubing somewhere in the loop...

Almost, but not quite - the sacrificial anode is going to get corroded quickly, that's its job after all, so if you used it as a length of tubing it would be the first bit to leak

As I understand it, the best bet would be to drop a lump of zinc into your reservoir, which would put it into electrolytic contact with the rest of the system, and make it easy to replace. Better yet, would be to attach it to a length of wire, and attach the other end to the metal bit that you want to protect.

For example:

You have a copper water block and an aluminium radiator. Hmmm - a problem. Because aluminium is more anodic than copper, galvanic corrosion will speed up the corrosion of the aluminium radiator, and slow down the corrosion of the copper block. Your radiator is going to rot more quickly than it would otherwise have done, and needs protecting.

There are two solutions:

1) Eliminate galvanic corrosion by making the system all-copper or all-aluminium. Now everything will corrode at it's normal rate.

2) Better yet, make galvanic corrosion workforyou. Drop a lump of zinc into your reservoir. Now the zinc is the most anodic metal in the system, which makes both the copper and the aluminium relatively cathodic (they're all connected by the water). This means that your disposable lump of zinc will corrode more quickly than it normally would, and both the copper and the aluminium (your imortant heatsink and radiator) will corrode more slowly.

Attaching the zinc to the aluminium by a wire will, if I have got this right, make the whole process more efficient by reducing the resistance between the cathode (aluminium radiator) and your sacrifical anode (waste zinc).

Edit: I've just been reading http://www.capsante.com/Articles/howto_sacrificial_zincs.htm, which strongly suggests that the electrical contact between the sacrifical anode and the cathode that it protects, is essential (although the belief that it rebuffs, suggests that other people may think it isn't...). I suspect it may be right, so I've done some edits to the article above to reflect the fact that there needs to be a wire between the waste zinc and the aluminium to be protected, as well as their being in the same electrolytic body (water).

_________________Charlie

Last edited by charliek on Thu Aug 12, 2004 7:18 am, edited 1 time in total.

Any ideas on how to do this without a reservoir? I'm looking to avoid a reservoir as a closed circuit with no standing water maintains full momentum and allows the pumps more efficient operation. This is why I was thinking of substituting a portion of the circuit tubing for zinc piping.

Any ideas on how to do this without a reservoir? I'm looking to avoid a reservoir as a closed circuit with no standing water maintains full momentum and allows the pumps more efficient operation. This is why I was thinking of substituting a portion of the circuit tubing for zinc piping.

Did you see my edit above, by the way?

One way might be to use a 'T' piece, like Chylld recommends, and to dangle a lump of zinc in it - you could push a wire through a pin-prick in it's sealing cap if you're using one, and seal around it. The other end of the wire would need to be attached (metal to metal so soldered or screwed somehow, ideally) to whichever of your metals is the most anodic (or all of them in a one-metal rig).

Another way might be to plumb in a 'sacrifical chamber' (hey, why not include a 'sacrifical altar' of some kind? Virgins, maybe... er... where was I?) Oh yes, another way might be to plumb in a 'sacrifical chamber' somewhere in the system, like a mini-reservoir, that will allow you to dangle in a sacrifical zinc anode and run a wire from it to your cathodic metals. The wire can be run outside the water, as long as the zinc is in the same water as the thing it seeks to protect, and the two metals have an electrically conductive contact between them.

On boats, it seems, they just bare the hull metal, bare a zinc ingot, and bolt one to the other.

You really need to bear in mind that the zinc is sacrificial, its going to be corroded on purpose, so don't do anything with it that's going to be upset if it vanishes!

Another thing to bear in mind is that there seem to be lots of folks who are sucessfully watercooling with little or no regard for any of this - so don't let it get too scary!

If the wire between sacrificial anode and cathode is essential, then I'd bet that the loop CANNOT be completed by the electrolyte (and requires a separate conductor).

As far as sacrificial anodes go, I'd be concerned about chunks of zinc breaking off and clogging the loop. The severity of this will depend on the block and a million other things, but I don't see a lot of potential for quiet pumps and effective filters.

If the wire between sacrificial anode and cathode is essential, then I'd bet that the loop CANNOT be completed by the electrolyte (and requires a separate conductor).

As far as sacrificial anodes go, I'd be concerned about chunks of zinc breaking off and clogging the loop. The severity of this will depend on the block and a million other things, but I don't see a lot of potential for quiet pumps and effective filters.

As I understand it, there will be a circuit through the electrolyte - that is part of what makes it an electrolyte - but it will be resistant, and the greater the distance between the metals, the greater the resistance.

The wire will have practically no resistance resistance by comparison, and therefore makes the anode/cathode relationship between the connected metals much stronger than any relationship established through the water.

After all, if there were absolutely no connection through the water, there'd be no galvanic corrosion.

As for bits of zinc breaking off, that may well be an issue. For that reason, one would want the anode to be somewhere where it is visible and easily changed. It will only be the surface of the anode that corrodes, so you would need to ensure that it was changed before it corroded right through at any point. That way you can minimise the risk of bits breaking off.

Having said that, an impeller pump system can handle a certain amount of particulate debris. Just ask Nemo.

As I understand it, there will be a circuit through the electrolyte - that is part of what makes it an electrolyte - but it will be resistant, and the greater the distance between the metals, the greater the resistance.

Hey, I'm just reading your link at Finishing.com. Sure looks like a watercooling loop to me. My reasoning is also based on the fact that the galvanic reaction is how a battery works. If you leave the battery disconnected from a load, it takes a truly long time to discharge, even though the anode and cathode are continually immersed in electrolyte. Same basic mechanism, right?

charliek wrote:

Which bit am I looking at? I just got a huge list of books.

Sorry, I've never figured out how to direct-link to a page with all that Javascript. Go to McMaster, put 2108 in the search box, and you'll find sacrificial anodes at the bottom of the page.

Just as a hypothetical to add to the 'is there a loop?' question... I do NOT know this to be factually true, and it might not be a factor, however think about the notion that we are always talking about loops and circuits when we are discussing a WC setup. I think it is quite reasonable to assume that one has two current paths in a WC setup, namely the waterline going TO the part and the line coming FROM the part. In addition we have a nice circulation of water to carry the corrosion products from one part to the next.

A way to test this might be to measure the ionic content of the coolant in different parts of the loop - if my theory is correct you would see a different ion mix (and polarity?) in the Al -> Cu section than you would in the Cu -> Al section.

Just as a hypothetical to add to the 'is there a loop?' question... I do NOT know this to be factually true, and it might not be a factor, however think about the notion that we are always talking about loops and circuits when we are discussing a WC setup. I think it is quite reasonable to assume that one has two current paths in a WC setup, namely the waterline going TO the part and the line coming FROM the part. In addition we have a nice circulation of water to carry the corrosion products from one part to the next.

While, in watercooling, there is generally a loop of some kind, for galvanic corrosion to happen there doesn't need to be one per se.

You can witness galvanic corrosion for yourself by those gimmicky 'lemon clock' or 'potato clock' things - you stick a copper rod and a zinc rod into a lemon (or potato) and there is enough current (galvanic potential) to drive a simple digital clock.

You only need one lemon, not a send lemon and a return lemon

This is because the ions in the different metals are basically whizzing between them - positive cations in one direction and negative anions in the other, through the same electrolyte.

Gooserider wrote:

A way to test this might be to measure the ionic content of the coolant in different parts of the loop - if my theory is correct you would see a different ion mix (and polarity?) in the Al -> Cu section than you would in the Cu -> Al section.

My guess would be that you would see the same type of traffic in both parts of the loop - maybe less of both in one direction than the other, due to increased resistance.

You could well be right Charlie. My theory is based on the idea that since you have a flow of coolant containing the particles put out by each source, the different blocks would in effect act as filters for the other block's particles. Sort of like if one substituted a pumped glass of lemonade in the example you cited - I would expect the current to carry the ions around the circle, and each block would suck up what was attracted to it, and spit out what wasn't so the ratios between the cation and anion would vary depending on where you measured them in the loop. (Not that it matters a great deal, or that I have the sort of equipment needed to measure the difference...)

In those lemon things, at the surface of the zinc or aluminium or whatever the more reactive metal is, atoms lose electrons to form positively charged metal ions (cations) in solution.

The electrons flow through the wire and clock and more wire to the other electrode. There, at the surface of the copper electrode, they react with something in the lemon juice or potato juice.

In principle it might be with oxygen and water to give negatively charged hydroxide ions (anions), or it might be with some positively charged ions (cations) to give something else, e.g. turning iron triple-positive to iron double-positive ions.

N.B. There is an electrical circuit involved here.

Also note that the overall reaction is between the zinc (which loses electrons) and some oxidising agent in solution (which gains them). The circuit and copper just make the process faster.

In a water-cooling system, the only candidate for the oxidising agent is oxygen from the air. So, when filling your system use water which has been boiled for a while to drive out the oxygen.

Finally: the shape of the water (loop or not) makes no difference. Gooserider, just as pumping the water keeps the temperature almost constant at all points, it will do the same for the concentrations of the various solutes.

A word of warning though: if you protect your anodic metal (by painting it, for example) and a small area of the paint is damaged, then you are left with an exposed anodic metal of much smaller surface area than the cathode, which means that it will corrode particularly quickly at the site of the coating damage (see Surface Areas above).

To avoid this problem, simply paint the cathode instead. That way you decrease the ratio of cathode/anode surface area, even if the paint chips or gets scratched off.

Painting the cathode leads to another problem... now I don't know a thing about watercooling, but won't a painted cathode have reduced heat transfer ability?

Sorry if somebody already mentioned this somewhere, but I looked through and didn't see it. Good post otherwise though. I'm no chemist or physicist, but I did take a materials science course nearly two years ago and we spent maybe a day or two on galvanic corrosion...

Unless I'm mistaken, and I may be, chemistry isn't my strong suit, having managed to somehow bypass it in highschool...

All that zinc has to go somewhere... it doesnt just 'disappear into the ether'... my guess is it's gonna get deposited onto your important bits of metal... how is THAT going to affect your heat transfer?

IMO, best solution... make every bit of metal in your watercooling loop, the same metal.. preferably the same grade... and use inhibitors. Replacing your liquid supply/refreshing the inhibitors frequently is probably a real good idea too.

Not that I'm against watercooling.. I'm certainly not.. been pondering it myself lately but.. all this is certainly food for thought, for staying with air cooled solutions.

Charliek+others:nice sharing of knowledge,and then,... is there a possibility for spcr to test/teach us Watercooling "KNOWHOW " :listing copper blocks family,silent copper pumps..... A french site tested pumps,PCSILENCIEUX,i think. Spcr may enlarge its specialty,including more water to its air

I agree with those stating that keeping the different metals insulated from each other (not counting the electrolyte) goes a long way in slowing down galvanic corrosion.
Just think of the wet cell batteries mentioned: As long as you don't connect the electrodes to any external circuit they will erode very slowly.

In water cooling, the main target for galvanic corrosion can be expected to be the joints where a cooling block (or radiator) of some metal (usually copper or aluminium) is attached to barbs/fittings of some other metal (usually brass or stainless steel). There you have a direct metal to metal contact as well as the electrolytic bridge!

An interesting read. Does anyone know exactly what the magnitude of difference galvanic corrosion makes to the overall corrosion rate?

e.g. Say for example we had a piece of aluminium of dimensions such that if put in water on its own 10% of it would corrode in a given time period, and we have a piece of copper that did the same, which would probably be of a different size.

If we put them in the water together how far is galvanic corrosion going to shift the 10%:10% balance in corrosion rates?

An interesting read. Does anyone know exactly what the magnitude of difference galvanic corrosion makes to the overall corrosion rate?

e.g. Say for example we had a piece of aluminium of dimensions such that if put in water on its own 10% of it would corrode in a given time period, and we have a piece of copper that did the same, which would probably be of a different size.

If we put them in the water together how far is galvanic corrosion going to shift the 10%:10% balance in corrosion rates?

The reaction rate is unaffected. The final concentration of the products is governed by equilibrium stoichiometry as described by the Nernst equation.

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